1. Field of the Invention
The present invention relates to communications, and more particularly, to a cell switching method and a cell switching system thereof.
2. Background of the Related Art
Generally, an asynchronous transfer mode (hereinafter abbreviated ATM) system preforms the steps of dividing user traffic into fixed-size packets, called cells, wherein destination information is placed into a 5 byte header, payload information is placed into 48 bytes, and the ATM system transfers the 53 byte cells to a destination.
Like telephone networks, ATM is a connection-based approach to communication. Thus, a fixed path (or virtual connection) is established from a point of origination to a destination by controlling the state of intermediate switches.
A protocol for transferring packet data in such an ATM system includes a physical layer, an ATM layer, an AAL (ATM adaptation layer), and an upper layer. The AAL is a layer for dividing and reassembling the packet data transferred from the upper layer, and is defined by types AAL1 to AAL5 (referring to ITU-TI series recommendation). AAL2 refers to variable bit rate service and AAL5 refers to connection-oriented data service.
The AAL2 protocol reduces the time required for packing each short user data into an ATM cell by multiplexing short packets transferred from various users on an ATM network over the same virtual connection, thereby optimizing the bandwidth of the ATM network. Namely, a basic concept of the AAL2 protocol is to multiplex or otherwise assemble various user data, which have short and variable lengths, into common part sublayer (CPS) packets.
FIG. 1 illustrates a block diagram of an AAL2 switch according to the related art. An AAL2 switch according to the related art includes AAL2 transceiver units 10 and 12 for demultiplexing or multiplexing the inputted AAL2 packet, and a switch 11 for switching an ATM cell at an ATM level.
An operation of the above-constructed AAL2 switch is as follows. First, an AAL2 receiving block in the AAL2 transceiver unit 10 demultiplexes the inputted AAL2 CPS packets into an ATM cell of 53 bytes. Thereafter, the AAL2 receiving block transfers the converted ATM cell to the ATM switch 11. Successively, the ATM switch 11 transfers the ATM cell to an AAL2 transmitting unit in AAL2 transceiver 12. Then, the AAL2 transmitting unit parses and multiplexes the ATM cell into AAL2 CPS packets so as to transmit the packets to a specified destination.
FIG. 2 is a block diagram for describing an AAL2 protocol recommended by ITU-T, according to the related art. An AAL2 protocol is divided into a Service Specific Convergence Sublayer hereinafter abbreviated SSCS) and a Common Part Sublayer (hereinafter abbreviated CPS).
Packet data of an upper application layer in the form of Service Data Units (hereinafter abbreviated SDU's) is transferred to an AAL layer through a Service Access Point (SAP). An AAL2 protocol SSCS then generates a SSCS-PDU (Protocol Data Unit) by adding a header and tail to the SDU.
Successively, the CPS generates a CPS-packet by adding a CPS-header to the SSCS-PDU (or CPS-SDU) transferred from the upper layer, and generates a CPS-PDU of 48 bytes by adding a start field to the CPS-packet. In this case, the CPS-packet becomes a payload of the CPS-PDU. As CPS-PDU comprises 48 bytes, CPS-packets of a plurality of users are multiplexed into the payload of a single CPS-PDU.
Subsequently, the CPS-PDU is transferred to an ATM layer. The ATM layer generates an ATM cell having a total size of 53 bytes by adding a 5 byte header of destination information to the CPS-PDU. Thus, the packet data received from the upper layer are divided/assembled into 48 bytes respectively through the AAL2 protocol so as to be used for the payload of the ATM cell.
The related art ATM switch and method thereof has various problems. For example, in order to switch the AAL2 CPS packets, the related art ATM switches convert the AAL2 cell into a ATM cell, switches the ATM cell to the corresponding destination, and then re-converts the ATM cell into AAL2 CPS packets again, thereby complicating the process, and resulting in processing delays. Moreover, the overhead of such an AAL2 switch degrades the Quality of Service (QoS) of a cell, reducing system efficiency.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.